Engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power outputs. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits. However, compressors are prone to surge. For example, when an operator tips-out of an accelerator pedal, an engine intake throttle closes, leading to reduced forward flow through the compressor, and a potential for surge. Surge can lead to noise, vibration, and harshness (NVH) issues such as undesirable noise from the engine intake system. In extreme cases, surge may result in compressor damage. To address compressor surge, engine systems may include a compressor recirculation valve (CRV) coupled across the compressor to enable rapid decaying of boost pressure. The CRV may recirculate compressed air from the compressor outlet to the compressor inlet.
One example of using a compressor recirculation valve to reduce surge is shown by Bjorge et al. in U.S. Pat. No. 8,739,530. Therein, the disclosed embodiment includes two compressors and a compressor recirculation valve coupled across each compressor. Each compressor recirculation valve is actuated open based on a desired flow rate through the respective compressor to avoid surge. The desired flow rate for each compressor may be calculated as a difference between throttle mass flow rate and mass flow through each compressor at a respective surge line on a compressor map.
The inventors herein have identified potential issues with such an approach. As one example, delays in actuating the compressor recirculation valves may lead to a slower than desired opening of the valves. During conditions such as an aggressive accelerator pedal tip-out, actuator delays may substantially reduce compressor flow rate and lead to compressor surge. Further, in an example when a throttle flow estimate is used to determine the desired compressor flow rate, errors in the throttle flow estimate may increase the likelihood of surge.
In one example, some of the above issues may be addressed by a method for an engine comprising: routing compressed air from a compressor through a throttle into an engine, diverting a portion of the compressed air away from the throttle through a recirculation valve to prevent the portion of the compressed air from flowing back into the compressor causing compressor surge, and diverting a further portion of the compressed air in response to a change in position of the throttle above a threshold change.
In another example, a method for a boosted engine comprises directing additional compressor recirculation flow from upstream of a throttle to a compressor inlet via a compressor recirculation valve, the additional compressor recirculation flow based on a filtered difference between a minimum desired compressor flow to reduce compressor surge and existing airflow through the throttle.
Thus, a compressor flow rate can be maintained above a flow rate at the surge line and compressor operation may be kept outside a surge region during transient engine operating conditions.
For example, an engine system may include a compressor having a compressor recirculation passage coupling an outlet of the compressor to the compressor inlet. In alternate embodiments, the recirculation path may couple an outlet of a charge air cooler to the compressor inlet. Flow through the recirculation path may be controlled via a continuously variable compressor recirculation valve (CCRV). An engine controller may be configured to continually adjust a position of the CCRV, during steady-state and transient engine operating conditions, based on changes in airflow through an intake throttle so as to maintain a compressor flow rate at or above a surge constrained flow rate (that is, a compressor flow rate at or above a surge limit of the compressor). During transient operating conditions (e.g. a sudden tip-out), the controller may increase an opening of the CCRV to direct an increased recirculation flow to the compressor inlet. The CCRV opening may be increased substantially only when throttle position undergoes a change in position that is higher than a predetermined threshold. Further, the increase in the opening of the CCRV may be based on a filtered difference between a minimum desired compressor flow to reduce compressor surge and existing airflow through the intake throttle. In one example, the filtered difference may be determined via a lead compensator.
In this way, by increasing recirculation flow through the compressor recirculation path during rapid transients, a compressor flow rate can be kept sufficiently high. This enables compressor operation to remain outside a surge region during sudden transients. By applying a filter to the difference between surge constrained flow rate and throttle mass flow via a lead compensator, a speed of response of the CCRV may be increased. Overall, surge margin under all engine operating conditions may be improved, and surge related NVH issues and component damage issues may be reduced. Further, engine performance and drivability may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.